As the first votes in the Republican presidential race approach, Rep. Ron Paul has become a serious force with the potential to upend the nomination fight and remain a factor throughout next year’s general-election campaign.

Although few think the congressman from Texas has a realistic shot at winning the GOP nod, he has built a strong enough base of support that he could be a spoiler — or a kingmaker.

In a muddled field, Paul could win the Iowa caucuses. While other candidates have been hesitant to commit to the state or have had trouble sustaining their initial bursts of support, Paul has been methodically building an organization and a growing corps of followers.

Over the past week, he has spent more than $600,000 on attack ads that are cutting into support for a fellow front-runner, former House speaker Newt Gingrich (Ga.). And Paul has built an organization that will allow him to remain in the race well beyond the early-voting states and amass convention delegates.

Perhaps most fearsome to Republican leaders is Paul’s refusal to rule out a third-party presidential bid that would steal votes from the Republican nominee and make President Obama’s path to reelection considerably easier.

A new Washington Post-ABC News poll, for instance, indicates that Obama and former Massachusetts governor Mitt Romney would be locked in a dead heat in a one-on-one contest. But in a three-way race with Paul, Obama would hold a wide advantage. The survey also suggests that Paul on his own would pose at least as much danger to Obama as Gingrich would.